Line an 8 × 8-inch square pan with parchment paper, leaving an overhang on the sides.
Reserve 1/4 cup of the peanut butter for the chocolate topping.
Combine the remaining peanut butter, graham cracker crumbs, powdered sugar, and melted butter in a large mixing bowl.
Stir until a thick, evenly combined mixture forms.
Transfer the peanut butter mixture to the prepared pan.
Press it very firmly into an even layer using the bottom of a measuring cup or glass.
Place the semi-sweet chocolate and reserved 1/4 cup peanut butter in a microwave-safe bowl.
Microwave in 20–30 second intervals, stirring thoroughly between each interval, until almost completely melted.
Continue stirring until smooth.
Pour the chocolate mixture over the peanut butter layer and spread evenly with a spatula.
Refrigerate for approximately 2 hours, or until firm.
Lift the chilled slab from the pan using the parchment paper.
Allow it to sit at room temperature for 5–10 minutes.
Cut into 16 bars using a sharp knife.
Store refrigerated until ready to serve.